The present invention relates to a method and apparatus for increasing the productivity of a direct reduction process for the production of metallized iron or other metal, and more particularly to apparatus and methods for increasing the amount of reductant in bustle gas of a direct reduction furnace while controlling the optimum bustle gas temperature and controlling the optimum bed temperature.
In the Midrex direct reduction process, reduction of iron oxides to metallized iron is accomplished by forming a bed of iron containing burden, such as iron oxide in pellet or lump form, in a shaft furnace injecting a heated reduction gas, typically a mixture of hydrogen and carbon monoxide, into the burden for a sufficient period of time to accomplish substantially complete reduction of the oxides to metallized iron. The reduction gas is typically injected into the burden using a bustle and tuyere system.
The problem presented is how to increase productivity of new and existing Direct Reduction furnaces without increasing the capacity of traditional reducing gas equipment.
The current state of the art to solve this problem is focused on four areas:
1) Enrichment Additionxe2x80x94for in-situ reforming within the reduction furnace;
2) Oxygen Injection to increase furnace bed temperatures for higher utilization;
3) High Enrichment Addition and Oxygen Injectionxe2x80x94primarily for in-situ reforming within the reduction furnace;
4) Oxy-fuel Burnersxe2x80x94to generate reductant outside of the furnace.
Each of the above items has some problems, which are described below:
1) Enrichment Additionxe2x80x94A hydrocarbon fuel (such as natural gas, methane, ethane, butane, propane, naphtha) is added to the bustle gas stream. The bustle gas methane can be controlled to prevent methanation. This methane is the chemical feedstock for in-situ reforming inside the furnace therefore increasing production by making more reductant. However, in-situ reforming consumes heat, so high amounts of enrichment addition reduce furnace bed temperatures. Lower bed temperatures result in less utilization of the reductant and a decrease in the kinetics of the reduction reactions. If bed temperatures become too low, then chemical equilibrium will favor the carburizing reactions 2COxe2x86x92[C]+CO2 and CO+H2xe2x86x92[C]+H2 O which consume part of the reductant and form oxidants.
2) Oxygen Injection is used in many Direct Reduction plants today, usually with low bustle gas methane levels (around 2.5%) to maintain bed temperatures around 900xc2x0 C., which is just below the clustering point of the burden material. The small amount of enrichment addition can be used to balance bed temperatures. High bustle gas temperatures allow furnace bed temperatures to be maximized. Higher bed temperatures increase the kinetics of the reduction reactions, increasing utilization of reductant, and yielding higher furnace productivity. A small amount of in-situ reforming can result from the enrichment addition. However, all equipment from the oxygen injection point and through the furnace are subject to the higher temperatures, with attendant higher risk of clustering of the burden, and higher maintenance requirements for the equipment. Oxygen injection is limited when the bustle gas temperature or furnace bed temperature is high enough to cause clustering of the material in the furnace. Much of the injected oxygen burns with the H2 and CO already present in the gas stream resulting in higher temperatures but less reductant and lower quality (since H2O and CO2 are produced). In short, current oxygen injection systems vary oxygen flow to control temperature and some systems vary natural gas flow to control methane levels without the ability to increase the amount of reductant generated or the quality of the gas stream. The quality of the reducing gas is defined by the ratio of reductants (H2+CO) to oxidants (CO2+H2O), the higher quality being the better. A typical value for the reductants-to-oxidants ratio is about 12 to 1, with a H2/CO ratio of 1.5 to 1.
3) High Enrichment Addition and Oxygen Injection. When a combination of oxygen injection and high enrichment addition is used, furnace bed temperatures are not maximized. Operating with high bustle gas methane (around 6%) maximizes in-situ reforming in the reduction furnace, with a consequence that the furnace bed temperatures are in the range of 800 to 820xc2x0 C. Production is increased, primarily by in-situ reforming. Some small gain in productivity may be realized by slight increases in bed temperatures through higher utilization and improved kinetics of the reduction reactions. Unfortunately, the extent to which pure oxygen and natural gas can be injected is limited by the high bustle gas temperatures which may cause clustering at the port tiles, that is, the point where the bustle gas enters the furnace. If the furnace bed temperature is low, then residual reductant in the top gas will be higher after the reduction reactions occur, and residual methane in the top gas will be higher after the in-situ reforming reactions occur, which results in a top gas with a higher heating value. When this heating value exceeds the heat needed by the reducing process, export fuel is generated. This increases the natural gas consumption of the process without providing a benefit other than export fuel.
4) Oxy-fuel Burners are unproven in DR plants. Oxy-fuel burners have the ability to generate reducing gas outside of the reduction furnace without significantly increasing bustle gas temperatures. The reductant generated is higher quality than can be achieved if the same amount of oxygen and natural gas were added through the typical oxygen injection+enrichment system. However, Oxy-fuel burners may not have the capability to produce reducing gas at the desired bustle gas temperature. The reducing gas generated by the oxy-fuel burners will not have the excess heat needed to maintain high furnace bed temperatures. For the oxy-fuel burners to produce high quality reducing gas, the ratio of oxygen to fuel must be controlled within precise limits. This ratio prohibits the oxy-fuel burners from supplying the temperature boost to the bustle gas that is required. If the oxy-fuel burner is operated at an oxygen/fuel ratio that is too high, then the burner can be destroyed by the high temperatures. Conversely, if the burner is operated at an oxygen/fuel ratio that is too low, then carbon could be formed, thus plugging the burner. It is possible that a larger number of oxy-fuel burners could be used to increase the bustle gas temperature, but this would prevent the plant from operating the burners to produce low quantities of additional reducing gas when operating at high furnace bed temperatures. Of course the need for more oxy-fuel burners to raise the bustle gas temperature would also require more capital investment.
The Midrex Direct Reduction Process is embodied in Beggs U.S. Pat. Nos. 3,748,120 and 3,749,386.
The invention provides a method and apparatus for increasing the productivity of a direct reduction process in which iron oxide is reduced to metallized iron by contact with hot reducing gas; comprising the steps of: a) providing a first hot reducing gas consisting essentially of CO and H2; b) providing additional reducing gas by reaction of a hydrocarbon fuel with oxygen; c) mixing the first hot reducing gas with the additional reducing gas to form a reducing gas mixture; d) enriching the reducing gas mixture by the addition of a gaseous or liquid hydrocarbon; e) injecting oxygen or oxygen-enriched air into the enriched mixture; and f) introducing the enriched mixture into an associated direct reduction furnace as reducing gas. Oxy-fuel burners supplement reducing gas flow.
The apparatus consists of a source of reducing gas communicating with a reduction furnace via a gas conduit, an oxy-fuel burner communicating with the gas conduit, means for injecting additional hydrocarbons into the gas conduit as enrichment gases, means for injecting oxygen into the gas conduit, and associated monitors, sensors, and controls.
The principal object of the present invention is to provide an improved method of increasing the amount of reductant in bustle gas of a direct reduction furnace while controlling the optimum bustle gas temperature and controlling the optimum bed temperature.
A further object of this invention is to provide apparatus to carry out the above method.
Another object of the invention is to increase the productivity of a direct reduction furnace without increasing the capacity or size of traditional reducing gas equipment.